Plasmonic-Driven Synthesis of Nanoprisms from Isotropic and Anisotropic Gold Cores

ABSTRACT

A nanoprism having a prismatic silver shell formed about a gold core and a process of forming the same are disclosed. The process includes irradiating a mixture of gold and silver nanoparticles with a narrow band of wavelengths capable of exciting the surface plasmon resonance of the gold.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 60/962,061, filed Jul. 26, 2007, which is incorporated herein by reference in its entirety.

STATEMENT OF GOVERNMENTAL INTEREST

This invention was made with government support under grant DMR-0520513 awarded by the National Science Foundation Materials Research Science and Engineering Centers, and under grant N00014-06-1-0079 awarded by the Office of Naval Research. The government has certain rights in the invention.

FIELD OF THE INVENTION

The invention relates to methods of forming nanoprisms from isotropic and anisotropic gold cores. In particular, the invention relates to methods of forming nanoprisms having a silver nanoprism shell formed about a gold core.

BACKGROUND

An interesting, unresolved issue in nanoscience involves the use of surface plasmon resonance (SPR) to effect nanocluster chemistry in a controllable manner. These electronic resonances have been studied for decades in the context of physical phenomena, such as plasmon coupling and wave-guiding (see Sanders et al., Nano Lett. 6, 1822 (2006)); and Jain et al., J. Phys. Chem. B. 110, 18243 (2006)), surface-enhanced Raman scattering (SERS) (see Hunyadi et al. J. Mater. Chem. 16, 3929 (2006); Wang et al., J. Am. Chem. Soc. 127, 14992 (2005); Qin et al., Proc. Nat. Acad. Sci. U.S.A. 103, 13300 (2006); and McLellan et al., Nano Lett. 7, 1013 (2007)), and electromagnetic field enhanced fluorescence (see Aslan et al., J. Am. Chem. Soc. 129, 1524 (2007); and Tam et al., Nano Lett. 7, 496 (2007)). In silver nanocluster synthesis (see Jin et al., Science, 294, 1901 (2001); Jin et al., Nature 425, 287 (2003); Bastys et al., Adv. Funct. Mater. 16, 766 (2006); and Xue et al., Ang. Chem. Int. Edit. 46, 2036 (2007)) observations have been made regarding the ability for surface plasmon excitation to control the growth of triangular prisms. It has been shown that silver nanoprisms grow under irradiation until their dipole plasmon resonance red-shifted to the excitation wavelength (see Jin et al., Nature 425, 487 (2003)). Thus, a need exists for core-shell structured nanoprisms and a process of forming the same using plasmonic driven synthesis.

SUMMARY OF THE INVENTION

Disclosed herein are nanoprisms having gold cores and silver prism shells and a process for forming the same.

One aspect of the invention is directed to a nanoprism having a gold core and a silver prismatic shell.

Another aspect of the invention is directed to a nanoprism having a gold core and a silver prismatic shell formed by irradiating a mixture of gold and silver nanoparticles with a narrow band of wavelengths capable of exciting a surface plasmon resonance of the gold nanoparticle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a transmission electron microscopy image of a nanoprism synthesized in accordance with an aspect of the invention, by irradiation with 550 nm light.

FIG. 1B is an extinction spectrum of the nanoprisms formed by a method in accordance with an aspect of the invention, after centrifugation.

FIG. 1C is a high-resolution transmission electron microscopy image of the {111} face of the nanoprisms formed by a method in accordance with an aspect of the invention.

FIG. 2 is a scanning transmission electron microscopy (STEM)—energy dispersive X-ray (EDS) analysis of a gold-silver nanoprism in accordance with an aspect of the invention. (A) STEM image of the nanoprism. (B) EDS spectrum of the silver nanoprism matrix (square spot), showing only silver signal. (C) EDS spectrum of the silver nanoprism matrix (circle spot), showing both a gold and a silver signal.

FIG. 3A is time-resolved extinction spectra of a mixture solution of 11 nm gold nanoparticles and 5 nm silver nanoparticles when irradiated with 550 nm light.

FIG. 3B is a transmission electron microscopy image of the product after irradiation of the mixture of FIG. 3A with 550 nm light.

FIG. 4 is a high-resolution transmission electron microscopy image of stacks of gold-silver nanoprisms in accordance with an aspect of the invention.

FIG. 5 is a transmission electron microscopy image of nanoprisms formed by a method in accordance with an aspect of the invention by (A) irradiation with 514 nm light and (B) 600 nm light.

FIG. 6 are transmission electron microscopy images of nanoprisms formed from (A) 5 nm gold nanoparticles and (B) 25 nm gold nanoparticles in accordance an aspect of the invention.

FIG. 7 is a schematic drawing showing the growth pathways for a nanoprism formed by a method in accordance with an aspect of the invention.

FIG. 8 are representative transmission electron microscopy images of nanoprisms in accordance with an aspect of the invention.

DETAILED DESCRIPTION

Disclosed herein are nanoprisms derived from gold nanoparticles and silver nanoparticles. The nanoprisms have a silver prismatic shell formed about a gold core (core-shell structure). The nanoprisms of the present invention have architectures that can be tuned by controlling excitation wavelength and core diameter.

The term “nanoparticle” as used herein refers to a metal composition that is, typically, less than about 1 μm in any one direction, but can be less than about 500 nm, less than about 200 nm, or less than about 100 nm. Alternatively, the nanoparticle can be up to about 5 μm. The nanoparticles can be, for example, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, and 100 nm.

The term “nanoprism” as used herein refers to a metal composition that exhibits prismatic properties. The nanoprism can have a single metal, such as a silver or gold nanoprism, but can also be a core-shell nanoprism, where a core metal has a prismatic shell. Thus, referring to FIG. 1A, a nanoprism in accordance with the invention has prismatic shell formed about a core (e.g., a core-shell structure). The core comprises gold, such as an isotropic gold nanoparticle, and the prismatic shell comprises silver, such as silver nanoparticles. Thus, the nanoprism has a gold core-silver prismatic shell structure. Referring to FIGS. 2B and 2C, this core-shell structure of the nanoprism can be confirmed using EDS. The gold core can be substantially spherical. Alternatively, the gold core can have a triangular prism shape. The silver prism shell can have, for example, a triangular or a hexagonal shape. The nanoprism can have an edge length of about 25 nm to about 500 nm, about 50 nm to about 400 nm, and about 100 nm to about 300 nm. The nanoprism can have an edge length, for example, of 25, 30, 35, 40, 45, 50, 55, 60, 65, 60, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, 250, 255, 260, 265, 270, 275, 280, 285, 290, 295, and 300.

Prismatic properties can be detected using known techniques. Prismatic properties include, but are not limited to, characteristic resonances. Referring to FIG. 1B, the nanoprism has a characteristic resonance at about 336 nm (corresponding to an out-of-plane quadrupole resonance), about 452 nm (corresponding to an in-plane quadrupole resonance), and/or about 632 nm (corresponding to an in-plane dipole resonance). The characteristic resonance of the nanoprism indicates that these gold core-silver shell nanoprisms can have optical features similar to optical features of pure silver nanoprisms without gold cores.

A nanoprism in accordance with the invention exhibits optical properties similar to pure silver nanoprism without a gold core. Use of silver nanoprisms is disclosed in U.S. Pat. Nos. 7,135,054 and 7,033,415, each of which is incorporated by reference in its entirety. The optical properties of the nanoprisms can be used in many biodiagnostic applications. The scattering properties of the nanoprism can be tailored by adjusting the size and shape of the nanoprisms, making the nanoprism useful as multicolor labels.

The nanoprism can be used as a diagnostic label, lighting up when target DNA is present. Biodetectors incorporating nanoprisms can be used to quickly, easily, and accurately detect biological molecules, as well as a wide range of genetic and pathogenic diseases, from genetic markers for cancer and neurodegenerative disease to HIV and sexually transmitted diseases.

Formation of Gold and Silver Nanoparticles

Gold and silver nanoparticles can be formed according to known methods. See Jin et al., Nature 425, 487 (2003); and Grabar et al., Anal. Chem. 67, 735 (1995). For example, gold nanoparticles can be formed by bringing an aqueous solution of a gold source, such as HAuCl₄, to reflux, and then adding trisodium citrate. The solution preferably is refluxed for about 15 minutes, and then allowed to cool to about room temperature. The gold colloid can be centrifuged and resuspended in trisodium citrate solution.

Alternatively, the gold nanoparticles can be formed by mixing an aqueous solution of a gold source, such as HAuCl₄, with trisodium citrate. A reducing agent, such as sodium borohydride (NaBH₄) then can be added to the mixture while stiffing vigorously. The solution preferably is allowed to age for about 3 hours, and then used as seed solution. The seed solution can be mixed with an aqueous solution of a gold source, such as HAuCl₄, and a stabilizer, such as poly(vinylpyrrolidone) (PVP). Ascorbic acid then can be added to the mixture and allowed to react for about 20 minutes to form the gold nanoparticles.

The gold nanoparticles can also be formed, for example, by bringing an aqueous solution of a gold source, such as HAuCl₄, to a boil, and then adding trisodium citrate quickly the boiling solution. The solution can be refluxed for about 15 minutes, and then allowed to cool to room temperature.

The gold nanoparticles can have a substantially spherical shape, or any other suitable shape. The gold nanoparticles typically have a diameter of about, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, and 50 nm. Isotropic gold nanoparticles typically have a surface plasmon resonance of about 500 nm to about 600 nm. For example, the gold nanoparticles can have a surface plasmon resonance of about 516 nm. The gold nanoparticles can also be prismatic, for example, the gold nanoparticles can be triangular nanoprisms. The surface plasmon resonance of triangular gold nanoprism is in the near-infrared region of the spectrum.

Silver nanoparticles can be formed, for example, by bubbling ice-cold deionized water with nitrogen gas in the dark and stiffing vigorously for about 30 minutes. A silver source, such as silver nitrate (AgNO₃), and trisodium citrate then can be added. An aqueous solution of a reducing agent, such as sodium borohydride (NaBH₄), can be rapidly injected into the solution. The reducing agent can be further added dropwise about every 2 minutes for about 15 minutes. The reducing agent and an aqueous stabilizer, such as bis(p-sulfonatophenyl) phenylphosphine (BSPP), then can be added dropwise. The resulting silver nanoparticle solution can be stirred gently for about 5 hours in an ice bath, and then allowed to stand overnight in the dark at about 4° C.

The silver nanoparticles can have a substantially spherical shape, or any other suitable shape. The silver nanoparticles typically have a diameter of about 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 nm. The silver nanoparticles typically have a surface plasmon resonance of about 395 nm.

Core-Shell Nanoprism Formation

Core-shell nanoprisms as disclosed herein can be formed by irradiating a mixture of gold and silver nanoparticles with a narrow band of wavelengths capable of exciting a surface plasmon resonance of the gold nanoparticle. The phrase “narrow band of wavelengths” as used herein refers to a specific wavelength plus or minus about 20 nm. In some embodiments, the narrow band of wavelengths has less than plus or minus 20 nm, e.g., plus or minus 15 nm, plus or minus 10 nm, or plus or minus 5 nm. Typically, a specific wavelength is selected by placing an optical filter between a light source and the object to be irradiated. This optical filter can have a width of about 40 nm, about 30 nm, about 20 nm, or about 10 nm.

The gold and silver nanoparticle mixture can have a ratio of gold to silver nanoparticles of about 1:5 to about 1:100; about 1:10 to about 1:50, and about 1:10 to about 1:20. The use of silver nanoparticles as the silver source keeps the concentration of silver ions (Ag⁺) low, but consistent throughout the photochemically induced Ag⁺ reduction and deposition onto the gold nanoparticle surfaces. Alternatively, AgNO₃ can be used as the silver source instead of silver nanoparticles, but the reaction and quality of the resulting nanoprisms are more difficult to control.

Irradiation can be performed for about 1 to about 100 hours, about 5 to about 50 hours, and about 10 to about 25 hours. For example, irradiation can be performed for about 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, and 100 hours. Irradiation is performed using a narrow band of wavelengths that is capable of exciting the surface plasmon resonance of the gold nanoparticle. For example, when a gold nanoparticle having a surface plasmon resonance of about 516 nm, irradiation can be performed using a narrow band of wavelengths of from about 450 to about 700 nm; from about 500 to about 650 nm; and from about 550 to about 600 nm.

Referring to FIG. 8, the nanoprism can have a gold prism core and a silver prism shell. Formation of nanoprisms having a prismatic gold core and a prismatic silver shell can be formed using gold nanoparticles having a prism shape and exhibiting a surface plasmon resonance in the near-infrared region. See Millstone et al., Adv. Funct. Mater. 16, 1209 (2006). For example, the gold nanoprisms can be triangular nanoprisms. A mixture of silver nanoparticles and gold nanoprisms can be irradiated in the near infrared region, for example, using a with a narrow band of wavelengths from about 750 to about 1400 nm; from about 850 to about 1200; and from about 900 to about 1100 nm.

Without intending to be bound by theory, it is believed that the growth of the nanoprism is attributed to plasmon excitation, and can be considered as a two-step pathway. Referring to FIG. 7, in the first step, the incident light preferentially excites the dipole plasmon resonance of the gold nanoparticles, which induces deposition of the silver layers. The silver ions are contributed from the dissolution of the silver nanoparticles. Preferential excitation of the gold nanoparticles occurs because the surface plasmon resonance of the gold nanoparticles is closer to the excitation wavelength than the surface plasmon resonance of the silver nanoparticles.

Without intending to be bound by theory, it is believed that the gold is behaving as a photocatalyst. The redox chemistry of Ag⁺ and citrate is thermodynamically downhill based upon their redox potentials (E_(Ag+Ag)=0.7996 V verses NHE; and E_(ADE, CO2/citrate)<−0.01 V at a pH above 8 (see Trettenhahn & Koberi, Electrochim. Act. 52, 2716 (2007)), and photoexcitation of a photocatalyst is not necessary to effect the redox reaction. AgNO₃ and citrate will react when boiling in the dark to form silver particles, carbon dioxide (CO₂), and 1,3-acetonediacarboxylate (ADE, the oxidation product of citrate). Photoexcitation of the particles may result in a catalyst that facilitates the reaction between Ag⁺ and citrate through photoinduced ligand rearrangement. It is unlikely that the process is through ligand dissociation because citrate and Ag⁺ are indefinitely stable (at least about 1 month) in the presence of gold nanoparticles. Furthermore, citrate is weakly bound to the surface of the gold nanoparticle. Therefore, it is unlikely that the particle is simply acting as a pre-catalyst activated by ligand dissociation, as one would expect to observe some background catalytic activity in the dark.

Referring to FIG. 3A, time-resolved UV-vis spectra demonstrates that as the gold core is coated with the silver shell during about the first 30 minutes of irradiation, the surface plasmon resonance band of the gold-silver nanostructure blue shifts from about 516 nm to about 500 nm. Referring to FIG. 3B, transmission electron microscopy analysis of the particles after the initial irradiation period demonstrates that the gold nanoparticles are covered by a silver shell with irregular shapes.

Referring to FIG. 3B and FIG. 7, during the early stage of nanoprism growth, the silver shell begins to exhibit an anisotropic morphology, but does not have the same morphology for each particle in solution. The early dispersity in nanoparticle architecture may be due in part to momentarily different electric field polarization across the nanoparticle surface during excitation, which could play a role in local redox chemistry. See Kelly, et al., J. Phys. Chem. B. 107, 668 (2003). As the excitation bands of the gold-silver particles approach the excitation wavelength, further excitation leads to the reconstruction of surface silver atoms into prismatic shells. The gold nanoparticles do not generally exhibit a change in the morphology. The growth of the silver shell does not exhibit dependence on the shape and symmetry of the gold cores. Without intending to be bound by theory, it is believed that the plasmon excitation leads to deposition of silver layers on the gold nanoparticle surface, and creates silver {111} twin planes. With continuous plasmon excitation, further silver shell growth accelerates in the direction of the parallel twin planes, while growth on the direction perpendicular to the {111} facet is much slower. Such a growth pattern can lead to three-fold symmetry which results in small triangular or hexagonal silver shells. Referring again to FIG. 3A, the growth of the silver shell stops when the dipole plasmon resonance band of the nanoprism red shifts with respect to the excitation wavelength (i.e. until the final nanoprism structure no longer absorbs the wavelength of irradiation).

Referring to FIG. 4, the side planes of the nanoprisms demonstrates that the number of twin planes varies, which could be due to the non-uniformity of the gold nanoparticle seeds. Hexagonal and triangular silver shells with different degrees of truncation are observed. Without intending to be bound by theory, it is believed that the varying degrees of truncation are structures that have not been fully transformed due to a depletion of the silver feedstock.

The nanoprisms have an average edge length of about 50 to about 100 nm. Referring to FIG. 5A, for example, when irradiation is performed using longer excitation wavelengths, such as about 600 nm, the nanoprisms have longer average edge lengths of about 75 to about 100 nm. Referring to FIG. 5B, when irradiation is performed using a wavelength that almost coincide with the plasmon band of the gold nanoparticles (for example, about 514 nm), the nanoprisms have average edge lengths of about 40 to about 60 nm. When irradiation is performed using a wavelength of about 550 nm, the nanoprisms have an average edge length of about 65 to about 80. Irradiation at wavelengths shorter than about 514 nm, such as about 488 nm, can lead to an increase in irregular anisotropic particles with a low yield of nanoprisms having the core-shell structure.

The nanoprisms have a thickness of about 2 to about 100 nm. The thickness of the nanoprism is dependent on the size of the gold nanoparticles used to form the nanoprism. Referring FIG. 6A, for example, formation of nanoprisms using gold particles having a diameter of about 2 to about 8 nm can lead to nanoprisms having a thickness of about 7 to about 10 nm. Referring to FIG. 6B, for example, formation of nanoprism using gold nanoparticles having a diameter of about 20 to about 30 nm can result in nanoprisms having a thickness of about 25 to about 45 nm.

Additional aspects of the invention will be apparent from the following examples, which are intended to be illustrative rather than limiting.

EXAMPLES Formation of 5 nm Gold Nanoparticles

Gold nanoparticles having a diameter of about 5 nm were prepared in accordance with known methods. An aqueous solution (about 20 mL) containing 0.25 mM HAuCl₄ and 0.25 mM trisodium citrate was prepared in a flask. Ice-cold, freshly prepared 0.1M NaBH₄ solution (about 0.6 mL) was added to the aqueous solution while stirring vigorously. The mixture was allowed to age for about 3 hours, and then was used as a seed solution. While stirring, about 2.5 mL of the seed solution was mixed with about 7.5 mL of an aqueous solution containing 0.25 mM HAuCl₄ and polyvinylpyrrolidone (0.1% PVP, MW about 58,000). Then, about 0.5 mL of 0.1 M ascorbic acid solution was added to the mixture and allowed to react for about 20 minutes while stirring. Transmission electron microscopy was used to determine the average diameter of the gold nanoparticles, which was about 5.3±0.8 nm.

Formation of 11 nm Nanoparticles

Gold nanoparticles having a diameter of about 11 nm were prepared in accordance with known methods. An aqueous solution (about 500 mL) of 1 mM HAuCl₄ was brought to reflux while stiffing. Fifty milliliters of 77.6 mM trisodium citrate solution was added quickly to the boiling aqueous solution. The mixture was refluxed for about an additional 15 minutes, and then allowed to cool to room temperature. The gold colloid was centrifuged at about 13.2 krpm (Eppendorf, 5415D) for about 30 minutes, and then resuspended in a 0.3 mM trisodium citrate solution. Transmission electron microscopy was used to determine the average diameter of the gold nanoparticles, which was about 11.2±1.8 nm.

Formation of 25 nm Nanoparticles

Gold nanoparticles having a diameter of about 25 nm were prepare by heating about 50 mL of an aqueous solution of HAuCl₄ (1% w/v) to a boil, while stiffing. Then, about 0.75 mL of trisodium citrate (1% w/v) was added quickly to the boiling mixture. The mixture was refluxed for about 15 minutes, and then allowed to cool to room temperature. The gold colloid was centrifuged at about 13.2 krpm for about 20 minutes, and resuspended in 0.3 mM trisodium citrate solution. Transmission electron microscopy was used to determine the average diameter of the gold nanoparticles, which was about 25.1±2.7 nm.

Formation of Silver Nanoparticles

Silver nanoparticles were formed by filling a three-neck flask with about 95 mL of NANORPURE™ water and immersing the flask in an ice bath. The water was bubbled with nitrogen gas in the dark with vigorous stirring for about 30 minutes. Then, about 0.5 mL of 20 mM AgNO₃ and 1 mL of 30 mM of trisodium citrate were added to the ice-cold solution. One milliliter of 50 mM NaBH₄ (freshly prepared with ice-cold NANORPURE™ water) was rapidly injected into the solution. Five to six drops of NaBH₄ solution was added to the mixture every 2 minutes for about 15 minutes. One milliliter of NaBH₄ solution and 1 mL of 5 mM bis(p-sulfonatophenyl) phenylphosphine (BSPP) solution were added to the mixture dropwise. The resulting silver nanoparticle solution was gently stirred for about 5 hours in an ice bath, and then allowed to stand overnight in the dark at about 4° C. Transmission electron microscopy was used to determine the average diameter of the silver nanoparticles, which was about 5.0±1.2 nm.

Formation of Gold-Silver Nanoprisms

About 9 mL of a solution of silver nanoparticles was mixed with about 1 mL of a solution of gold nanoparticles that was diluted to have an optical density of about 1 O.D./mL at extinction maximum. The solution was irradiated with a 150 W halogen lamp coupled with an optical bandpass filter (Intor, Inc.). When the optical bandpass filter was centered at about 550±20 nm, triangular nanoprisms having an average edge length of 70±6 nm were formed. When the optical bandpass filter was centered at about 514±20 nm, triangular nanoprisms having an average edge length of about 48±5 nm were formed. When the optical bandpass filter was centered at about 600±20 nm, triangular nanoprisms with an average edge length of about 80±7 nm were formed.

The foregoing describes and exemplifies the invention but is not intended to limit the invention defined by the claims that follow. All of the methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the materials and methods of this invention have been described in terms of specific embodiments, it will be apparent to those of skill in the art that variations may be applied to the materials and/or methods and in the steps or in the sequence of steps of the methods described herein, without departing from the concept, spirit, and scope of the invention. More specifically, it will be apparent that certain agents which are both chemically and physiologically related may be substituted for the agents described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those of ordinary skill in the art are deemed to be within the spirit, scope, and concept of the invention as defined in the appended claims 

1. A nanoprism comprising a gold core and a silver prismatic shell.
 2. A nanoprism having a gold core and a silver prismatic shell produced by a process comprising: irradiating a mixture comprising at least one gold nanoparticle and at least one silver nanoparticle with a narrow band of wavelengths capable of exciting a surface plasmon resonance of the gold nanoparticle.
 3. The nanoprism of claim 1, wherein the nanoprism has a surface plasmon excitation resonance hand of about 500 nm.
 4. The nanoprism of claim 1, wherein the nanoprism has a characteristic resonance at one or more of about 336 nm about 452 nm, and about 632 nm.
 5. The nanoprism of claim 1, wherein the gold core is substantially spherical.
 6. The nanoprism of claim 1, wherein the gold core is a prism.
 7. The nanoprism of claim 6, wherein the gold core is a triangular prism.
 8. The nanoprism of claim 1, wherein the silver prismatic shell is a triangular prism.
 9. The nanoprism of claim 1, wherein the nanoprism has a thickness of about 2 to about 50 nm.
 10. The nanoprism of claim 1, wherein the nanoprism has an average edge length of about 50 to about 300 nm.
 11. The nanoprism of claim 2, wherein the mixture is irradiated for a time of about 1 to about 100 hours.
 12. The nanoprism of claim 2, wherein the ratio of gold to silver nanoparticles is about 1:5 to about 1:100.
 13. The nanoprism of claim 2, wherein the gold nanoparticle has a diameter of about 2 to about 50 nm.
 14. The nanoprism of claim 2, wherein the gold nanoparticle has a diameter of about 2 to about 8 nm, and the nanoprism has a thickness of about 7 to about 10 nm.
 15. The nanoprism of claim 2, wherein the gold nanoparticle has a diameter of about 20 to about 30 nm, and the nanoprism has a thickness of about 25 to about 45 nm.
 16. The nanoprism of claim 2, wherein the silver nanoparticle has a diameter of about 1 to about 10 nm.
 17. The nanoprism of claim 2, comprising irradiating with a narrow band of wavelengths of about 500 nm to about 700 nm.
 18. The nanoprism of claim 2, wherein the narrow band of wavelengths comprises wavelengths of about 600 to about 700 nm and the nanoprism has an average edge length of about 75 to about 100 nm.
 19. The nanoprism of claim 2, wherein the narrow band of wavelengths comprises wavelengths of about 500 to about 525 nm and the nanoprism has an average edge length of about 40 to about 60 nm.
 20. The nanoprism of claim 2, wherein the narrow band of wavelengths comprises wavelengths of about 530 to about 580 nm and the nanoprism has an average edge length of about 65 to about 80 nm.
 21. The nanoprism of claim 2, wherein the narrow band of wavelengths comprises wavelengths of about 750 to about 1400 nm and the gold core is a prism.
 22. The nanoprism of claim 21, wherein the gold core is a triangular prism. 